Currently, the highest yield strength linepipe commercially available is about 80 ksi. While higher strength steel has been experimentally produced, e.g., up to about 100 ksi several problems remain to be addressed before the steel can be safely used as linepipe. One such problem is the use of boron as a component of the steel. While boron can enhance material strength, steels containing boron are difficult to process leading to inconsistent products as well as an increased susceptibility to stress corrosion cracking.
Another problem relating to high strength steels, i.e., steels having a yield strength greater than about 80 ksi, is the softening of the HAZ after welding. The HAZ undergoes local phase transformation or annealing during the welding induced thermal cycles, leading to a significant, up to about 15% or more, softening of the HAZ as compared to the base metal.
Consequently, it is an object of this invention to produce low alloy, ultra high strength steel for linepipe use with a thickness of at least 10 mm, preferably 15 mm, more preferably 20 mm, having a yield strength at least about 120 ksi and a tensile strength of at least about 130 ksi while maintaining consistent product quality, substantially eliminating or at least reducing the loss of strength in the HAZ during the welding induced thermal cycle, and having sufficient toughness at ambient and low temperatures.
A further object of this invention is to provide a producer friendly steel with unique secondary hardening response to accommodate a wide variety of tempering parameters, e.g., time and temperature.